This internal function models a covariate using a "bounded normal" distribution by first standardizing the covariate values to the range [0, 1], noninclusive, then fitting a generalized linear model (GLM) under the Gaussian family function.
covparams: List of vectors, where each vector contains information for one parameter used in the modeling of the time-varying covariates (e.g., model statement, family, link function, etc.).
covlink: Vector of link functions.
covname: Name of the covariate at index j.
obs_data: Data on which the model is fit.
j: Integer specifying the index of the covariate.
model_fits: Logical scalar indicating whether to return the fitted models. The default is FALSE.